rstp bpdu

[sarahr202]

Hi everybody

Please consider the following questions:

1)Does a switch in rstp transmit bpdu out of all its port i.e root port and alternate port in stable network? ( for e.g in stp, bpdu are sent out of designated ports . Root port and blocked port only receives bpdu in stable converged stp network)

2) In stp, a switch only transmits bpdu if it receives one from its root bridge . When it does receive one, it changes the sender bridge id, sender port and root path cost before transmitting it to its neighboring switch.

My book says in rstp a switch transmits bpdu even it does not receive one from its root bridge.

Here is my question.

Suppose we have switches all running rstp. one switch say sw2 does not receive bpdu from its root switch say sw1. According to my book, sw2 still sends bpdu to its neighboring switch say sw3. How  doesthis bpdu differ from the one sw 1 originally transmitted to sw3 when sw2 received a bpdu from sw1 i.e root bridge?

Thanks and have a great weekend.

[Reza Sharifi · Accepted solution]

Hi Sarah,

Every bridge that receives the RLQ, checks the Root Bridge ID in the

query and performs either of the following:

o If this Root Bridge ID matches the current root information stored

locally, the bridge relays the RLQ upstream, across its root port.

o If the bridge receiving the RLQ is the root bridge, it floods a positive

RLQ response out of ALL its designated (downstream) ports. In our

example, this is the case, and “A” immediately responds

o If the bridge receiving the RLQ has different root bridge information

other than one found in RLQ, it immediately responds with a

negative RLQ, flooded out of all designated (downstream) ports.

Have look at this doc for more info"

http://blog.ine.com/wp-content/uploads/2010/04/understanding-stp-rstp-convergence.pdf

Have a nice weekend

HTH